NASA Technical Reports Server (NTRS) 19630010727: FLIGHT-AND FORCE-TEST INVESTIGATION OF A MODEL OF AN AERIAL VEHICLE SUPPORTED BY TWO UNSHROUDED PROPELLERS

FLIGHT- AND FORCE-TEST
INVESTIGATION OF A MODEL
OF AN AERIAL VEHICLE SUPPORTED
BY TWO UNSHROUDED PROPELLERS

by Robert H. Kirby;

Langley Research Center,

Langley Station, Hampton, Va.

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION • WASHINGTON, D. C. • SEPTEMBER 1963

TECHNICAL NOTE D-1965

FLIGHT- AND FORCE -TEST INVESTIGATION

OF A MODEL OF AN AERIAL VEHICLE SUPPORTED

BY TWO UNSHROUDED PROPELLERS

By Robert H. Kirby

Langley Research Center
Langley Station, Hampton, Va.

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION

TECHNICAL NOTE D-I965

FLIGHT- AND FORCE-TEST INVESTIGATION
OF A MODEL OF AN AERIAL VEHICLE SUPPORTED
BY TWO UNSHROUDED PROPELLERS
By Robert H. Kirby

SUMMARY

An investigation of the static and dynamic stability and control character-
istics in hovering and at low forward speeds has been made on a small-scale flying
model of an aerial vehicle supported by two unshrouded propellers that were fixed
with respect to the airframe so that the propeller plane of rotation was horizon-
tal for hovering flight. The model in its basic configuration consisted of a box-
like body in the center, with the two propellers mounted in tandem on struts in
front of and behind the body and guard rings around the propellers.

The investigation showed that in hovering, the controls -fixed pitching and
rolling motions of the model were unstable oscillations. Since the periods of the
oscillations Were relatively long, however, the model could be controlled fairly
easily in hovering without artificial stabilization. In forward flight, the basic
model required an increasing nose-down attitude for drag trim as the forward speed
was increased and became very difficult to control longitudinally at speeds above
21 knots, mainly because of increasing static longitudinal instability of angle
of attack. For reasonably satisfactory stability and control characteristics in
forward flight, and particularly for speeds above 21 knots (38- knots, full scale),
horizontal and vertical tails were required.

INTRODUCTION

The National Aeronautics and Space Administration has investigated simplified
models of a number of configurations that might be suitable for a light, general-
purpose VTOL aerial vehicle. As originally visualized, these vehicles would be
able to hover or fly forward at speeds up to about 60 knots and would carry a pay-
load of about 1,000 pounds. Basically, they consist of a body for the engine,
pilot, and cargo supported by two or more propellers that are either shrouded or
unshrouded. The propeller plane of rotation is horizontal for hovering flight
and, for most configurations, is fixed with respect to the airframe.

The results of an investigation of a l/ 3-scale model of a vehicle having two
fixed shrouded propellers are reported in references 1 and 2, and the results of

a similar investigation of a model with four shrouded propellers are reported in
reference 5« Two rather serious problems brought out in these tests which seem
inherent in any simple shrouded-propeller configuration in forward flight are an
undesirably large nose-down pitch attitude required for trim at the higher speeds
and a nose-up pitching moment which increases rapidly with increasing forward
speed.

One approach to the problem of excessive nose-down pitch attitudes required
for higher speeds is to tilt the shrouded propellers with respect to the airframe.
Reference 4- gives the results of an investigation of a model that had three
shrouded propellers in a triangular arrangement, one in front and two at the
rear, that could be tilted with respect to the airframe.

Another approach to the problem of the undesirable pitching-moment and pitch-
attitude characteristics of the fixed shrouded-propeller configurations is the use
of unshrouded propellers because of the smaller pitching moment and drag resulting
from translational velocity. References 5 and 6 give the results of an investiga-
tion made with a model which had four unshrouded propellers that were fixed with
respect to the airframe so that the propeller plane of rotation was horizontal
for hovering flight.

The present investigation was made with a model which had two unshrouded pro-
pellers in tandem that were fixed with respect to the airframe so that the pro-
peller plane of rotation was horizontal for hovering flight. This paper presents
the results of a series of free-flight tests and static force tests performed in
the Langley full-scale tunnel to obtain the static and dynamic stability and con-
trol characteristics of the model in hovering and in forward flight. The flight-
test results were obtained mainly from pilots' observations and from studies of
motion-pi cture records of the flights.

SYMBOLS

The longitudinal forces and moments were determined with respect to the wind
axes and the lateral forces and moments were determined with respect to the body
axes. The axes originated at the center of gravity of the model.

c chord of horizontal tail, in.

F l lift force, lb

Fp drag force, lb

Fy side force, lb

My pitching moment, ft-lb

My rolling moment, ft-lb

Mrz yawing moment, ft-lb

2

My variation of pitching moment with angle of attack, ft-lb/deg

(X

MYy variation of pitching moment with forward speed, ft-lb/knot

variation of side force with angle of sideslip, lb /deg
variation of rolling moment with angle of sideslip, ft-lb/deg
variation of yawing moment with angle of sideslip, ft-lb/deg

it horizontal- tail incidence relative to fuselage axis, positive when

trailing edge is down, deg

a angle of attack of fuselage axis relative to horizontal (tilt angle),

deg

P angle of sideslip, deg

APPARATUS AND TESTS

Model

The basic model is shown in the photograph of figure 1 and the sketch of
figure 2. The model was a simplified research vehicle that was not intended to
represent any specific full-scale machine but the size was such as to represent
approximately a 0.3-scale model of proposed full-scale machines. The model was
designed to have the same size cargo box and the same width as the earlier models
in references 1, and 5*

The model propellers were of laminated-wood construction and had fixed blade
angles of 13° at 0.75 radius. The propellers were driven through gearboxes and
interconnecting shafting by a pneumatic motor which was controlled by a throttling
valve. The propeller guard rings were intended to protect the propellers without
appreciably affecting the propeller characteristics and therefore were made of
relatively small diameter tubing and located so as to provide a large tip clear-
ance. The center of gravity of the model was at the geometric center of the model
and in the plane of the propellers.

Figure 3 shows the horizontal- and vertical-tail surfaces that were added to
the basic configuration. The horizontal tails had an airfoil shape and were
mounted outboard of the propeller guard rings. The vertical tail was a flat plate
and was mounted under the rear half of the rear propeller.

For all of the tests the model control moments (pitch, roll, and yaw) were
provided by small compressed-air jets located at the side and rear of the model as
shown in figure 3* These jet-reaction controls were operated by the pilots who
controlled them remotely through the use of flicker-type (on or off) electro-
pneumatic actuators. The actuators were equipped with integrating- type trimmers

3

which trimmed the control a small amount in the direction the control was moved
each time a control deflection was applied. With actuators of this type, a model
hecomes accurately trimmed after flying a short time in a given flight condition.

The flicker- control moments used during the tests were about ±17 foot-pounds
in pitch, ±6 foot-pounds in roll, and ±9 foot-pounds in yaw. Total travel on the
pitch jet-reaction control (flicker control plus trim) provided ±28 foot-pounds
of moment which resulted in ±11 foot-pounds of pitch trim being available before
a reduction of flicker control occurred.

The weight and mass characteristics of the model varied somewhat from one
phase of testing to another, as tails, ballast weights, etc., were added or
removed. The following values are felt to be reasonably representative of aver-
age values for the model and varied not more than ±10 percent during the tests.

Weight, lb 52

Moment of inertia about pitch axis, slug-ft^ 4.1

Moment of inertia about roll axis, slug-ft2 1.6

Moment of inertia about yaw axis, slug- ft 2 5.6

Tests and Testing Techniques

Flight tests .- The flight tests were made to determine the dynamic stability
and control characteristics of the basic model in hovering flight in still air
and in forward flight up to a model speed of about 33 knots (60 knots, full
scale). In addition, horizontal- and vertical-tail surfaces were added to
improve the stability and control characteristics at the higher forward speeds.

Figure 4 shows the test setup for the flight tests made in the Langley full-
scale tunnel. The sketch shows the pitch pilot, the safety-cable operator, and
the thrust controller on a balcony at the side of the test section. The roll and
yaw pilots were located in an enclosure in the lower rear part of the test sec-
tion. All of these operators were located at the best available vantage points
for observing and controlling the particular phase of the motion with which each
was concerned. Motion-picture records were obtained with fixed cameras mounted
at the side and at the upper rear of the test section.

The air to drive the propellers and for the jet-reaction controls was sup-
plied to the model through flexible plastic hoses, and the power for the electric
control solenoids was supplied through wires. These wires and tubes were sus-
pended from overhead and taped to a safety cable of l/l6-inch aircraft cable from
a point approximately 15 feet above the model down to the model. The safety
cable, which was attached to the model at the center of gravity, was used to pre-
vent crashes in the event of a power or control failure or in the event that the
pilots lost control of the model. During flight the cable was kept slack so that
it would not appreciably influence the motions of the model during the normal
course of the tests.

The test technique is best explained by describing a typical flight. The
model hung from the safety cable with the tunnel airspeed at zero, the model

4

power was increased until the safety cable became slack and the model was in
steady hovering flight. The tunnel drive motors were turned on and the airspeed
began to increase. As the airspeed increased, the pitch pilot applied nose-down
control and trim to tilt the model to the required attitude and the power opera-
tor adjusted the power to the model propellers in order to provide the thrust
needed to balance the lift and drag of the model and to keep the model as near as
possible to the center of the test section. Flights were also made in which the
airspeed was held constant at intermediate speeds so that the stability and con-
trol characteristics at constant speed could be studied.

Hovering-f light tests were made with rhe same technique and setup except
that the tunnel test section was not needed nor used. The tests were performed
in a large enclosed area (one of the return passages of the Langley full-scale
tunnel) which provided protection from random disturbances due to wind and was
large enough to reduce the slipstream recirculation effects to negligible values.

Force tests .- Force tests were made to determine the static stability and
control characteristics of the model for correlation with the flight-test results.

The model was secured, through an internal six- component strain- gage balance,
to /a portable sting and strut support system. The model and support assembly was
then installed in the 30- by 60 -foot test section of the Langley full-scale tunnel.
The static longitudinal characteristics of the model were investigated by setting
a tunnel speed and then covering a range of angles of attack from 0° to -35° at a
constant model propeller speed. Normal force, axial force, and pitching moment
were recorded at each test point. Such tests were made at each of several tunnel
speeds in a range from 0 to 30 knots. The longitudinal characteristics were
investigated for the basic configuration and for the basic configuration with
horizontal-tail surfaces added at incidence angles i-^ from 20° to 40°.

The static lateral characteristics of the model were investigated for angles
of sideslip between 20° and -20° at angles of attack between 0° and -30°. For
each angle of attack investigated, the tunnel speed was adjusted to give zero drag
for an angle of sideslip of 0°. The effect of a vertical- tail surface mounted
under the rear half of the rear propellers was investigated. In addition, the
effect of the horizontal tail on the lateral characteristics was also determined.

No wind-tunnel corrections have been applied to the data since the model is
very small in proportion to the size of the tunnel. Since conventional aerody-
namic coefficients lose their significance and tend to become infinite as the air-
speed approaches zero, the results of the force tests are presented in dimensional
form. The model used in this investigation was constructed primarily for the
flight tests and the construction techniques used were not well suited for high-
power runs for extended periods of time as required in force testing. The force
tests, therefore, were run at reduced model power. Except for the basic longi-
tudinal data, the forces, moments, and velocities presented in this report have
been scaled up to correspond to the flying weight of the model.

5

RESULTS AND DISCUSSION

Hovering Flight

In hovering flight the model had unstable oscillations in both pitch and
roll (as has been the case for the models reported in references 2, l 5 , and 6),
the rolling oscillation being somewhat more difficult to control because of its
greater instability. Time histories of typical controls-fixed pitch and roll
oscillations, obtained from motion-picture records of model flights, are pre-
sented in figures 5 and 6, respectively. The approximate periods and damping of
these oscillations, as measured from these records, were:

Pitch Roll

Period, sec 5*25 3*50

Time to double amplitude, sec 1.55 0.80

Time to double amplitude, cycles 0.30 0.23

In spite of the instability of these oscillations, the model could be controlled
fairly easily in hovering flight, particularly in pitch, mainly because the
periods of the oscillations were fairly long and the control power was adequate.
The controllability of the present model in roll was found to be better than for
any of the models reported in references 2, 3 j or 6. The shrouded-propeller
models of references 2 and 3 were extremely difficult to control in roll without
artificial stabilization because the oscillations had very short periods, were
very unstable, seemed to be predominantly angular motions, and were very easily
excited by translational movement or horizontal gusts. The model of reference 6,
which had four unshrouded propellers, was much easier to control by remote con-
trol but did have a tendency to translate or "slide" considerably as a result of
very little change in angle of roll. This tendency resulted in the model being
somewhat difficult to fly steadily or to stop at an exact spot after a maneuver.
The present unshrouded model was a little easier to fly in roll or to position
accurately than the model of reference 6 although it still required careful pilot
attention. At times, the pitch pilot could demonstrate the controllability of
the model by letting the pitching oscillation build up and then apply control to
stop the oscillation. The roll pilot, however, could not always stop an oscilla-
tion if he allowed it to develop.

A few hovering-f light tests were made with the tail surfaces shown in fig-
ure 3 installed on the model. There was no noticeable difference in the pitching
motion of the model. In roll, however, the tails made the model a little easier
to control, probably because of the increased damping and inertia of the hori-
zontal tails.

No difficulty was experienced in controlling the model in yaw. As might be
expected, the model was neutrally stable about the yaw axis in hovering and could
be controlled easily for the very limited conditions covered in the tests - flight
in still air and maintaining a given heading as the only task for which the yaw
control was used.

6

Forward Flight

The basic longitudinal data from the force tests are presented in figure 7
arid a summary of the model's static longitudinal characteristics is shown in fig-
ure 8. The basic lateral data from the force tests are presented in figure 9 and
a summary of the model's static lateral characteristics is shown in figure 10.
These data will be discussed in the following sections along with the results of
j the model flight tests.

Longitudinal characteristics . - The flight tests showed that as the forward
speed increased, the basic model without tails required an increasing nose-down
moment for pitch trim and an increasing nose-down attitude for drag trim. At
j 11 knots (20 knots, full scale) the model required about 11 foot-pounds of nose-
down pitch trim and, as the speed increased, additional pitch trim was required
at the expense of the flicker- control moment available in this direction.

Finally, at a speed of about 21 knots (38 knots, full scale), the model became
very difficult to control and experienced fairly rapid pitch-up divergences. The
pilot believed that this condition was caused by two factors. First, the trim
i requirement was so great that there was only about one-half the nose-down control
left to arrest the nose-up motion. Secondly, as the forward speed increased, the
model seemed to have an increasing static longitudinal instability of angle of
attack. To check on the reduced control factor, the available pitch trim was
increased by 9 foot-pounds which again gave the pilot about the full ±17 foot-
j pounds of flicker control at 21 knots. With this increased control power, the
model could be controlled more easily at 21 knots and flights were made to about
26 knots, but at this speed the pitch pilot again lost control of the model.

Figure 8 presents a summary of the tilt -angle a and pitching-moment varia-
I tions with forward speed for the basic model and for the model with horizontal
j tails installed. The data for the basic model show the increasing nose-down

attitude required for drag trim and the increasing pitching moment and increasing
static longitudinal instability of angle of attack with forward speed. The data
show that at 21 knots, the basic model required 17 foot-pounds of pitch trim and
had a static attitude instability of 0.^5 foot-pound per degree of angle-of-
attack change. The data further show that there was no appreciable increase in
j pitch trim required above 21 knots and that the static longitudinal instability
increased to a value of about 0.60 foot-pound per degree of angle -of -attack
change at about 25 knots and stayed at this value with increasing speed.

Even though the force test data indicate no increases in the static insta-
bility or longitudinal trim above 26 knots, the model could not be flown above
this speed even with the increased pitch control. The probable reason for this
was that with a given level of static longitudinal instability the normal acceler-
ations resulting from a given angular divergence became so large with increasing
speed that it was too difficult to fly the model in the tunnel test section at
speeds above 26 knots.

In order to improve the behavior of the model at the higher speeds,
horizontal- tail surfaces (shown in fig. 3) were installed on the model. Most of
the flight tests were made with a tail incidence of about 25° • With the tails
installed the model motions were very smooth and the model was easy to fly up to
30 knots which was the highest speed tested. At speeds above 20 knots, the model

7

did not exhibit the pitch-up tendencies of the basic model and the pitch trim
requirements were reduced. The flight tests showed, however, that the model with
tails installed did have a very mild dynamic instability. When the pilot
refrained from giving control (controls fixed), the model developed a gentle
unstable oscillation of fairly long period, somewhat like a phugoid oscillation.

In general, both the flight and force test results showed that horizontal
tails having variable incidence would be required to obtain the optimum stability
and trim throughout the speed range tested. Since the model had to cover an
attitude range a of 0° to 30 °, it was not possible to keep the tails unstalled
and lifting in a positive direction with any one angle of incidence. For example,
the data of figure 8 show that with 20° incidence the tails were probably
unstalled and made the model stable over most of the tilt-wing range (a = -10° to
- 30 °), but at tilt angles greater than -20° the tails set at this incidence pro-
duced more nose-up pitching moment My than the basic model. On the other hand,
with ^0° incidence, the tails made a greater contribution to trim but did not
make the model 'stable with attitude except at speeds greater than 28 knots.

Lateral characteristics .- The most noticeable lateral characteristic of the
model in forward flight was its tendency to sideslip. As the forward speed
increased, the model became difficult to keep exactly alined with the wind and,
if allowed to sideslip, was difficult to straighten out. The pilot felt that,
at best, the model had about neutral directional stability. Since the yawing
motions affected the rolling motions to some extent because of the dihedral
effect of the model, this characteristic became objectionable to the pilots at
forward speeds of around 15 knots (27 knots, full scale) and above.

A vertical tail, mounted under the rear half of the rear propeller as shown
in figure 3* was installed on the model to improve the directional stability;

This tail gave adequate directional stability and made the lateral motions very
easy to control. Figure 9 gives the basic lateral data and figure 10 presents a
summary of the static lateral characteristics of the model with and without the
vertical and horizontal tails installed. These data show agreement 'with the
flight-test results in that the basic model had neutral directional stability and
the vertical tails gave a significant improvement. The horizontal tails gave an
additional increment of directional stability at the higher forward speeds.

In roll, the basic model was about as easy to fly in forward flight as it
was in hovering up to speeds of about 15 knots which was the highest speed tested
without the tails installed. This result was in contrast with the results
reported in reference 2 in which the ducted-propeller tandem configuration expe-
rienced an increasing dynamic instability in roll with increasing forward speed.
With the vertical and horizontal tails installed, the model was fairly easy to
fly in roll over the entire speed range of the tests (up to 30 knots).

The data of figure 10 show that the basic model had positive effective
dihedral -My^ over most of the speed range. Adding the vertical-tail surface

below the rear propeller added an increment of negative effective dihedral but
the further addition of the horizontal tails gave about the same results as the
basic model.

8

CONCLUSIONS

On the "basis of a static and dynamic stability and control investigation in
the Langley full-scale tunnel on a model which had two unshrouded propellers in
tandem that were fixed with respect to the airframe , the following conclusions
were drawn:

1. In hovering, the controls -fixed pitching and rolling motions of the model
were unstable oscillations. In spite of these oscillations, the model could be
controlled fairly easily in hovering without artificial stabilization mainly
because the periods of the oscillations were relatively long.

2. In forward flight, the basic model required an increasing nose-down atti-
tude for drag trim as the forward speed was increased. The model experienced an
increasing nose-up pitching moment and static longitudinal instability of angle
of attack with 'increase in forward speed up to about 23 knots. No appreciable
increases were experienced above this speed.

3. The basic model became very difficult to control longitudinally at speeds
above 21 knots, mainly because of the static longitudinal instability with angle
of attack.

Horizontal tails were required for reasonably satisfactory longitudinal
stability and control characteristics in forward flight. Variable tail incidence
was also required because of the large tilt angles experienced by the model.

5. The basic model had about neutral directional stability in forward flight
and became difficult to control at speeds above 15 knots.

6. With a vertical tail installed under the rear propeller, the model had
satisfactory directional stability and was easy to control over the speed range
of the tests.

Langley Research Center,

National Aeronautics and Space Administration,

Langley Station, Hampton, Va. , June 25* 19&3*

9

REFERENCES

1. Parlett, Lysle P. : Wind-Tunnel Investigation of a Small-Scale Model of an

Aerial Vehicle Supported by Ducted Fans. NASA TN D-377> I960.

2. Parlett, Lysle P. : Stability and Control Characteristics of a Small-Scale

Model of an Aerial Vehicle Supported by Two Ducted Fans. NASA TN D-920,
1961.

3. Parlett, Lysle P. : Stability and Control Characteristics of a Model of an

Aerial Vehicle Supported by Four Ducted Fans. NASA TN D-937> 1961.

4-. Smith, Charles C. , Jr.: Wind-Tunnel Investigation of a Small-Scale Model of

an Aerial Vehicle Supported by Tilting Ducted Fans. NASA TN D-409, i960.

5. Kirby, Robert H. : Force-Test Investigation of a Model of an Aerial Vehicle

Supported by Four Unshrouded Propellers. NASA TN D-123^, 1962.

6. Kirby, Robert H. : Flight-Test Investigation of a Model of an Aerial Vehicle

Supported by Four Unshrouded Propellers. NASA TN D-1235, 1962.

11

Figure 1.- Photograph of basic model. L-62-2^76

Figure 2.- Drawing of basic model. Jet reaction controls and tails not shown.

All dimensions are in inches.

L-61-2837

Figure 4 .- Typical setup used for forward-flight tests in the Langley full-scale

tunnel .

Pitch angle , deg Horizontal displacement, ft

Time , sec

Figure 5.- Typical controls-fixed model pitching oscillations in hovering. Basic

model .

15

o

□

o

A

k

k

Q

O

a , deg
0
-5
-10
-15
-20
-25
-30
-35

(a) No horizontal tails.

Figure 7.- Basic longitudinal data. Vertical tails on.

17

a , deg

O 0

□ -5

O -10

a -15

-20

b -25

Q -30

-35

Figure 7*- Continued.

18

a , deg

0

0

□

-5

0

-10

A

-15

k

-20

b

-25

D

-30

O

-35

(c) Horizontal tails on; i^ = 30°.
Figure 7 .- Continued.

19

Tail

i, , deg

Off

On

20

On

30

On

40

Forward speed knots

Figure 8.- Variation of longitudinal characteristics with forward speed for various

tail incidences. Zero drag.

21

o

□

o

A

t\

&

D

a , deg

Speed , kn

0

0

-5

13.4

-10

18:6

15

26.0

-20

31.9

-25

36.5

-30

42.4

15 20

(a) No tails.

Figure 9 .- Basic lateral data. Zero drag at p = 0°.

22

(b) Vertical* tail on.
Figure 9 *- Continued.

23

Tails off
Vertical tail on

Vertical and horizontal tails on

Figure 10.- Variation of lateral characteristics with forward speed with and

without tails.

NASA -Langley, 1963

l-5^91

25